In petrochemical refinery, acid-alkaline wash is carried out in the process of petroleum processing and product refining. Alkaline wash is to treat the petroleum products with a certain concentration of sodium hydroxide, which will remove impurities such as thiols, phenols, thioethers, naphthenic acids in the petroleum products, while the exhausted lye after wash becomes spent caustic wastewater. The COD values of the spent caustic wastewater are usually particularly high, up to hundreds of thousands. Sulfides, phenols and other substances in spent caustic wastewater are highly risky and toxic compounds, which are seriously harmful to both animals and plants. Direct wastewater discharge would not only be toxic to aquatic animals and plants, but also consume large amounts of oxygen dissolved in water, causing aquatic animals and plants to lose their habitats. Therefore, spent caustic wastewater must be effectively treated; otherwise it will cause serious damage to the environment.
At present, the methods for treating petrochemical spent caustic wastewater mainly include incineration process, oxidation-reduction process, supercritical water oxidation process, wet air oxidation process, mild wet air oxidation process, catalytic wet air oxidation process, biological method, ultrasonic method and so on, wherein catalytic wet air oxidation process is the most widely used among all the oxidation processes. Catalytic wet air oxidation process is to oxidize and decompose organic matter and ammonia in wastewater into harmless substances such as CO2, H2O, N2 by air oxidation under the action of certain temperature, pressure and catalyst, so as to achieve the purpose of purification. Although the method can decrease the COD value of spent caustic wastewater effectively, due to high temperature and high pressure in wastewater treatment processes, the energy consumption of wastewater treatment is large and the requirement for the treatment equipment is high.